In the manufacture of chromium chemicals from chromite ore by the conventional alkali roasting process, chromite ore containing about 40 to 50 percent chromic oxide (Cr.sub. 2 O.sub. 3) is drymilled to fine particle size, mixed with finely divided sodium carbonate, lime and leached residue from previous operation, and is roasted at temperature in the order of about 2000.degree.F. in an oxidizing atmosphere to oxidize the chromic oxide to form water-soluble hexavalent chromium compounds. The roast is then leached to extract the water-soluble chromium compounds, leaving a residue for which there is presently no use, and which generally is disposed of by dumping. In spite of use of efficient leaching methods, the residue still contains minor amounts of water-soluble chromium compounds which will bleed therefrom when the residue is wetted, as by exposure to rain. Such bleeding is objectionable because the soluble chromium compounds are toxic and raise a serious pollution problem. Some of the residual water-soluble chromium compounds in the residue have relatively low degree of solubility and bleed very slowly, hence cannot be readily leached to exhaustion. They nevertheless have sufficient solubility to pollute the environment by bleeding long after the residue has been discarded. Such bleeding can pose severe problems of ground water pollution. To stockpile these residues under conditions that they could not be wetted would be difficult and expensive.
The slowly bleeding chromium compounds contained in the residue obtained in the manufacture of chromium chemicals from chromite ore include calcium chromate, CaCrO.sub.4, and calcium alumino-chromate, 3CaO.Al.sub.2 O.sub.3.CaCrO.sub. 4. 12H.sub. 2 O, which are very slowly soluble in water; tribasic calcium chromite, Ca.sub. 3 (CrO.sub. 4).sub.2, which decomposes slowly in the presence of water to produce water-soluble hexavalent chromium and insoluble trivalent chromium hydroxide; and basic ferric chromate, Fe (OH)CrO.sub. 4, which hydrolyses slowly in water to release chromate ions. The waste residue also contains some trivalent chromium compounds, but these are soluble to lesser extent only and are not major contributors to pollution. These residual, slowly soluble chromium compounds generally are present in amount of from about 0.7 to 1.5 percent by weight of chromium. In addition, the residue usually contains minor amounts of soluble alkali metal chromate which was not completely removed in the leaching operation. The residue, when wetted, may bleed chromium salts to the extent that the aqueous bleed can contain up to thousands of ppm of chromium compounds, expressed as Cr, depending on the relative amounts of water and residue, efficiency of contact, and time of contact.
The prior art has attempted to solve the chromium bleeding problem by reducing the hexavalent chromium compounds to form trivalent chromium compounds which bleed only to negligible extent on exposure to water. Usually, this involved roasting the residue in the presence of a reducing agent. A number of reducing agents have been suggested for this purpose, such as coal, gaseous hydrocarbons, sulfuric acid or sulfuric pitch containing hydrocarbon waste material and heavy waste oil. It has also been proposed to use chemical reducing agents which do not require roasting to reduce the chromate in the residue, such as ferrous sulfate or spent hydrochloric pickle liquor. While effective, all of these methods require use of additional reagents, and, in the case of the reductive roasting operation, require additional amounts of fuel.